Semiconductor devices with backside power rail and backside self-aligned via

A semiconductor structure, comprising:

• a power rail on a back side of the semiconductor structure;
• a first interconnect structure on a front side of the semiconductor structure;
• a source feature, a drain feature, multiple channel layers, and a gate structure that are between the power rail and the first interconnect structure, wherein the multiple channel layers connect the source feature and the drain feature, and the gate structure wraps around each of the channel layers;
• a first source silicide feature disposed on a back side of the source feature and between the source feature and the power rail, wherein the first source silicide feature is also disposed on sidewalls of the source feature that extend from the back side of the source feature towards the front side of the semiconductor structure;
• and
• a first via connecting the first source silicide feature and the power rail.

A method comprising:

• providing a structure having a substrate, a stack of first semiconductor layers and second semiconductor layers that are alternately stacked layer by layer over the substrate, a sacrificial gate structure disposed over a channel region of the stack, and gate spacers on opposing sidewalls of the sacrificial gate structure;
• etching the stack adjacent the gate spacers to form a source trench and a drain trench on both sides of the sacrificial gate structure;
• epitaxially growing a source feature in the source trench and a drain feature in the drain trench;
• forming a first dielectric layer on side surfaces of the source feature, side surfaces of the drain feature, side surfaces of the source trench, and side surfaces of the drain trench;
• and
• forming a contact etch stop layer (CESL) over the first dielectric layer, the source feature, and the drain feature, wherein a first air gap is sealed between the CESL and the first dielectric layer in the source trench and a second air gap is sealed between the CESL and the first dielectric layer in the drain trench.